Apparatus for measuring the quality of material



Feb. 26, 1946. R. W. OSBORNE 2,395,425

APPARATUS FOR MEASURING THE QUALITY OF MATERIAL Filed July 31, 1942 3 Sheets-Sheet 1 ATTORN R. W. OSBORNE Filed July 31, 1942 APPARATUS FOR MEASURING THE QUALITY OF MATERIAL Qww 1 m wkQ w \MXQ R h f w\\ m PM I l np/ l 0 ll I W. A A m A I In hi W A W 3 mx M, Q21 5 \mfl ag ww b a h mw v A v v 3 Q m w w 3 MW & Q w fiowx x Feb. 26, 1946.

Feb.26,1946.

R. W. OSBORNE APPARATUS FOR MEASURING THE QUALITY OF MATERIAL Filed July 31, 1942 3 Sheets-Sheet 3 and mm 46506252 kfyslem flrz aye Mfwork Patented Feb. 26,1946

UNITED STATE s PATENT OFFICE A assure lrrana'rus Foa MEASURIhlG m QUALITY or MATERIAL Ralph Wiilon'ghby Osborne, Toronto, Ontario,

Application Iulyil, 1942, Serial No. 452,994

' 7 Claims. (01. 175-183) preferably by some means of continuous automatic controls Many materials, particularly liquids such as sulfuric acid, show variations in electrical characteristics such. as conductivity with variations in quality (1'. e., concentration), and these electrical characteristicsmay therefore be parture of this indicating device mm the zero or null ,point., The indicator thusregisters only employed as a basis for controlling the quality of the material.

It has been proposed-heretofore to use the electrical conductivityof a good conducting material suchas sulfuricacid as a basis for controlling its quality, Alternating current has generally been used in measuring the conductivity of an electrolyte to avoidthe decomposition of material accompanying the passage of direct.v current therethrough. In order to maintain decomposition at a minimum, it is preferable to employ high frequency alternating current such as is produced by a vacuum tube oscillator circuit for determining variations in electrical characteristics such as conductivity of materials of this type.

Particularly, when conductivity electrodes such as those described in Patent 2,296,867, issued on my co-pending application Serial No. 289,924, filed August 12, 1939, are employed, which electrodes are designed .to admit electricity to a liquid for determining its conductivity without having the electrodes in contact with the liquid, so as to avoid attackon the electrodes by a corrosive material such as sulfuric acid, it is important that a high frequency alternating current produced,

for example, in a vacuum tube oscillator, be employed to make effective the electrostatic coupling a diii'erential between the characteristics of the standard side and the test side of the bridge network. This type of measurement, 1. e., the

null or differential method, is considered desirable in that the device may be so designed that variations in electrical characteristics having, no relation to the variation in quality of the material being tested. i.' e., due to changes in atmospheric conditions such as temperature, have substantially an equal eifect on both sides of the bridge network and accordingly cancel out in their eii ect on the differential indicator.

When the testing of a corrosive or easily decomposed chemical product makes the useof high frequency alternating current desirable in controlling the quality of the material inaccordance with its electrical characteristics, disadvani tages are noted in employing abridge network in which onelhigh frequency, circuit associated with the standardmaterial is balanced against another high frequency circuit associated with the materialbeing tested, One disadvantage noted is that the two high frequency" circuits, since they are operating simultaneously in close proximity, even though well shieldedwill interi'ere to some extent if operated at 'or near the,

same frequency, and one of the conditions of obtaining a satisfactory nuli point and difl'er'en tial reading is that the circuits in balance should be operated at asnear the same frequency as possible. Another disadvantage noted when two vacuum tube oscillator circuits are employedin a bridge network is that as the vacuum tubes in bridge system, comprising two high frequency circonductivity measurements heretofore cults balanced against each other, suitable for obtaining accurate differential indications of variation froma standard in the quality ofa I chemical product that is corrosive or is susceptible to electrolytic decomposition.

It is another object of this invention to provide a bridge system comprising two vacuum tube oscillator circuits for controlling the quality of a corrosive or easily decomposed chemical product, in which circuits there :are means tending to make the vacuum tube characteristics change with greater uniformity as the tube elements age so that less frequent adjustment of null point in the bridge network will be necessary.

two high frequency circuits making up the bridge network is intermittently in operation, and the system is so designed thatthetwo high frequency circuits are never in operation simultaneously, but always alternately. Theintermittent operation of the two circuits may be accomplished,

for example, by using two oscillator circuits to make up the bridge system, and exciting these two circuits by means of a low frequency A. C. source of power, in such a way that each of the .two oscillators is excited by the alternate half,-

cycles of the low frequencyA. C. source of'power.

The frequency of alternate excitation,i. e., the frequency of the A. C.'powcr source used for excitation, should be sufficiently high that an dicating current, load, or similar circuit characteristic does not show fluctuations corresponding to the alternate periods of operation and nonoperation of -the}circuit, but instead, due to the mechanical inertia of the instrument, gives a steady indication of load or current or similar characteristic in lthatcircuit. Any convenient low frequency, e. g., a commercial A. 0. power source within the usual frequency range of 25 to 60 cycles per second, may be employed. 1 l

have foundit generally practicable to use an exciting current (i. e.,'-A. C. 'powersource) of frequency in the range 25 to 600 cycles per second, though in particular instances frequencies outside this range may be employed. In the embodirneht shown in the drawings, the A. C. power sourcehas a frequency of 60 cycles.

, This A. '0. power source may advantageously ings, through a variable resistance, or through the coil in an indicator responsive to current or load in that oscillator circuit, or through both anindicator coil and a variable resistance, and the other end of each secondary may be connected to an anode in a vacuum tube, preferably each secondary being connected through an induct-l .anceto one of theitwo platesin a twin triode, as shown in the drawings. When one of the more complex oscillator circuits referred to above is used, tetrodes or pentodes, or twin tubes of these types, may be employed, and an electrode other than the plate may be charged by the A. 0. power source. As indicated above, the secondaries are oppositely wound; i. e.,going from the ground connection tothe anode of the tube, the two secondaries are wound in opposite directions so that each of thetwo anodes is positively charged on alternate half cycles of the exciting A. C. voltage. Since the oscillator .circuit operates only when the plateior other charged electrode) has a positive charge, it is evident the two oscillator circuits will operate on respective halfcycles of the exciting voltage; When one oscilcapacitative coupling to the 'dead circuit, but

since the dead circuitfhas no exciting current When the vacuum tube is of the heated cathode type,. as shown in the drawings, the power for. the cathode heaters may be derived from an 7 the prim of power.

As above indicated, the use a twin vacuum 1,.

tube, 1. e. a'tube in which two complete sets of electrodes are contained within a single envelope,

tubewere in operation simultaneously at or near the same frequency, the inter-electrode capaci- 10 ties within the twin tube would cause serious interference between the two oscillator circuits. However, since in the bridgesystem of my invention only one of the oscillator circuits is in operation at any instant the capacitativeooupling between the electrodes of thecircuit-in opera electrical instrument in the excited circuitintion and-the electrodes of the dead circuit does not affect the indicator, as above pointed out; the twin tube construction is therefore practicable in my'bridge system. As abovepointed out, in a twin type of tube, thetube characteristics of'eachset of elements change with substantially complete uniformity with aging of the tube; this, of course, is not the case with separate vacuum'tubes. Accordingly, with thetwin type of tube, little or no adjustment of the network is required during the-life of the tube.

Since it is important in a control system for comparing the qualityof a process material with a standard material that the bridge system should so be in constant accurate balance, it is seen that my control system using thetwin type of tube is particularly advantageous. i i

It should be understood, howeverythat the use of a twin vacuum tube is merely an additional advantageous feature of my invention andthat,

if desired, separatevacuum tubes" may be employedin each oscillator circuit instead of the twin tube. Withsuch a design alternate excitation of the two oscillator circuits in accordance with my invention would still have the advantage of eliminating interference'betweeri the two-cir+ cults. 1 a

In the bridge network, one of the oscillatorcircuits has as part of its load acircuit element, the

impedance of which dependsat least in partupon which depends upon an electrical characteristic of the material in process. The circuit element, the impedance of which is dependent upon an electrical"characteristic ofeither the standard or process material, may beja conductor formed 85 of the material, for example, a column of sulfuric acid, as in Patent 2,296,867, issued on my copending case Serial No. 289,924 referred to above, or as in United States Patent 933,015 of August 31, 1909.

so The circuit element containing'the standard or process' materialfis preferably inserted directly in theoscillatory circuit so that the load on the oscillator circuit depends upon its resistance as a conductor in theoscillator circuit. Such a conlator circuit is operating and thelother is dead, struction is shown in Figure 1 of the drawings some energy will be transferred by inductive or If desired, however, the standard and process materials may each be contained in separate circuits and each of these separate circuits associated through inductive coupling with one of the 0scil-- lator circuits in the bridgenetwork. Such a tie-f sign is shown in Figure 3 of the accompanying drawings. Another method of associating the circuit elements containing the standard and process materials to the oscillator circuits is by electron additional secondarywindingbn the transformer" 7 coupling, in which oscillation 'frequency I the circuit rather than dielectrics.

willbe mmmumy'mamnaem oi variations'in load. .This may be accomplished. for example, by employing tetrodes in the oscillator circuits, using the screen ofsuch tube instead r the plate for connection-to the tank-circuit of the oscillator, and placing the load in a separate circuit from plate to cathode. However, as above stated, -I prefer inserting the elements containing standard or process material as conductors in the plate circuits, as shownin Figure l of the drawings The circuit element; containing the standard material or the material in process, particularly in the case of acpnducting liquid such as sulfuric acid. is preferably a core orcolumn of the material acting as a resistance in the circuit contain ing it, as shown inhPatent 2,296,867, issued on my copending applicationand in-Patent 933.0115, re

ferredto above. As shown in Patent 2,296,867,1s-

sued-on mycopending application, a corrosive in electrical characteristics othe than conductivity. For example, when the material is a non conductonthe circuit element containing it may be a condenser-in which the material is the dielectric..-,; In this case, variation in the quality ofthe material results in variation in the reactance of the circuit element containing it, which in turn reflects variation in the load on theoscillator circuit in which it is connected. The element containing the process material as dielectric thus acts as avariable condenser in the circuit, while the element containing the standard materialvas dielectric acts-as a fixed condenser. A fixed, resistance, which serves to load the oscillator, is advantageously connected in series with suchelement. In this method of operation, 1. e., using the process and standard materials as dielectrics in condenser units, the variationsin capacityof thecircuit element containing the process material serving as dielectric cause variations in the load on that oscillator, and also cause variationsin theirequency of the oscillation.

I. have found it advantageous to employ the process and standard materials as conductors in have found it advantageous to design the circuits so that theoscillator frequencies remain substantially'constant andso that the twocircuits in the bridge system operate ator near the same frevquency, since change in the frequency of an oscilrectconnection therein as resistance elements (Figure 1),.or, alternatively, through inductive coupling with the oscillator circuits (Figure 3).

. These circuit elements thus, in either case, provide impedance in the oscillator circuits with which theyare associated. Variations in the quality of the process material would therefore cause variations in the impedance provided by the circuit element containing it in its associated oscillator circuit, thus changing the load on this oscillator circuit. The load on the oscillato circuit associated with the element containing the standard material remains substantially constant Further, I

elements containing mans under constant atmosphericconditions. The load on eachof the oscillator circuits may be measured by the current takenby each of these circuits fromthesource of power. U: i

. In'order to cancel out the effect of v a taken by each circuit due to changing atmospheric' conditions such as temperature, which conditions should affect each of the two circuits substantially identical1y, it is advantageous'to get an indication of the difference in load, i., e;, di flferencein current flowingin the two circuits rather than the absolute value of thiscurrent. in either circuit. Accordingly, t is advantageous to use an indicator responsive to the diiferencein load on the two oscillators. Thismay, for examf ple, be a direct currentmilliammeter with middle zero scale, as shown inliigure 1,connected.across corresponding points in the two oscillator circuits so' asto show the unbalance due to change in characteristic of product material. Alternative ly, the differential indicator may: bean instrument containing two coils, eachconnected in series in one of the oscillator circuits, the two coils tending to produce opposite torques 'on an element of the "indicator; such an instrument ,isshown in Figures 2 and 3. In this arrangement, the full currents of the oscillatorcircuits pass through the indicatorcoils.

,The indicator may be adjusted to a null reading ata point where the material in process has a desired quality. It is preferable to employ a standard material having; substantially the same quality as that desired in the material in process so that changes in atmospheric conditions such as temperature will have substantially identical effects on the electrical characteristicson the standard material and the material in process and variations in load, due to thesevariations in atmospheric conditions will therefore cancel out in the indicator. Such adjustment (to obtains null'reading) ispreferably made by variable re sistances associated with the indicator instrument, as shownparticularly in Figures 1 and 2,

or, alternatively, by variable condensers shunted from each'vacuum tube. anode tocathode, as shown inFigure 3. Also, where the oscillator gcircuits involve inductive coupling, as in Figure 3, this coupling may be varied to bring the differential indicator of the bridge system to a null reading. These resistances, condensers, or inductances mayalso be adjusted to increase the sensitivity of the system to slight changes in quality of the process material.

In the preferred arrangement .for obtaining a differential indication of load on the two oscillator circuits and for adjustment of the indicator in quality (and the load on the oscillator withwhich it is associated varies -accordingly),the

-bridge system previouslyin balancebecomes unbalanced and a difference of potential is produced :between. the ends of thetwo secondary gcoils across which the milliammeter is shunted.

This difference of potential is shown by. a deflection in the milliammeter. Thisarrangement for indicating differential oscillator load and adjustloads.

vantageous since the full use of the available energy of the oscillator circuits makes it unnecessary to use a delicate instrument; i. e., it

is possible to use a comparatively inexpensive instrument of rugged construction.

The use of variable resistances orpotentiometers associated with the indicator (Figures 1 and 2) I have found to be preferable to the use of variable condensers or variable inductive couplings in the tank circuits of the oscillators (Figure 3) for adjustment of the indicator to a null reading. The arrangements involving variable resistances and potentiometers associated with the indicator do not involve adjustment of elements in the high frequency portion of the oscillators, which would affect the frequency of oscillation. As above stated, change in frequency of an oscillator affects the sensitivity of that oscillator to change in resistance of the product material which is associated with that oscillator cirstandard material, preferably of substantially the same quality as the desired quality of the material in process. The circuit elements containing each of these materials, 1. e., the standard material and the material in process, are associated each to one of the oscillator circuits, preferably by being directly connected in the oscillator circuits, but in alternative methods by inductive coupling, electrode coupling or other means of association with the oscillator circuits. The two oscillator circuits are excited alternately, the frequency of excitation of each circuit being sufflciently rapid that an instrument indicating current in that circuit, for example, would not pulsate. The frequency of alternate excitation, however, is sufficiently low so that in each excitation there is sumcient time for a large number of oscillations in the circuit being excited. A differential indication of the load on the two oscillator circuits is obtained. As the quality of the material in process departs from the desired cult and this is undesirable since precision of temperature compensation depends on equality in sensitivity of the two oscillator circuits.

As variations in quality of the process material are shown by the indicator readin process conditions may be adjusted accordingly to correct the quality of the materiah This may alsobe accomplished automatically. For example, the indicator may contain electrical contacts for actuating process equipment to alter the quality of the material in process in accordance with indications from the control device. In controlling the admixture of water and concentrated sulfuric acid, to produce sulfuric acid of a desired concentra tion, for example, the above device may be employed, as shown in the drawings, and electrical contacts may be provided on either side of the standard. the load on its associated oscillator circuit varies, the differential indicator deflects and actuates process equipment for correcting the quality of the material in process to the desired standard. After one actuation of the process equipment for correcting the quality of the material in process, a time lag of from a few seconds to several minutes is preferably introduced before another actuation. of the process equipment is possible.

On the accompanying drawings. there are shown for purposes of illustration preferred embodiments of my invention. It willbe noted that the several elements of the illustrated device, as

indicator needle whereby upon deflection of the needle a motor is actuated to open or close a valve controlling the addition of water. The indicator may be designed so that the magnitude of deflection controls the period of operation of a valve-controlling motor, i. e., the extent to which the valve is opened and closed.

In order to avoid hunting, the actuating apparatus connected to the indicator may be so designed that, after operation of the valve-conw trolling motor or other adjustment controlling the quality of the material in process, a time lag is introduced whereby a period varying from a few seconds to several minutes will elapse before the indicator will again be effective to actuate the process equipment controlling the quality of the material in process.

The method of controlling the material in process in accordance with my invention thus involves ment, preferably resistance, such that it forms part of the load of an oscillator circuit. This oscillator circuit is balanced in a bridge network against another oscillator circuit which has as may be represented diagrammatically either in their actual physical form or in the form of their equivalent electric circuit elements. So far as possible, the same reference character has been employed for each of the elements throughout the various figures of the drawings, in whatever form the element is represented.

Reference numeral I indicates a twin triode of the heated cathode type. This tube has twin cathodes 2, 2 with a common connection in the .baseof the tube; these cathodes are heated by the twin heater circuits 3, 3 in series which derive their power from an auxiliary low-voltage secondary winding 4 in a transformer, the pri-- mary winding 5 of which is connected to an A. C. power source of 220 volts, cycles, indicated at 6. The twin triode I has two plates I and I, each of which is connected to the resonant tank circuit of an oscillator circuit. Plate I is connected through an inductance 9 to a secondary winding III in the transformer having the primary winding 5. The plate 8 of the twin triode I is connected through the inductance II to the secondary winding I2 of the transformer which has the primary winding 5. The other ends of secondary windings Wand I2 are connected to and I2 are in opposite directions;' accordingly,

' plates 1 and 8 are positively charged on alteremploying the material as part of a circuit ele- 7o fcompletes this plate circuit to the cathode 2 of vacuum tube I. The plate circuit of plate I in part of its-loads circuit element comprising a addition to inductance II includes acondenser II which completes this circuit to the cathode 2 of vacuum tube Cathodes 2, 2 are grounded as indicated at l3. In the vacuum tube the grid I3 is associated with plate 1, and the grid 22 with plate 3. The circuit of grid I9 includes in series the grid leak and condenser 23 and the coil 2| completing the circuit to the cathode 2. The circuit of grid 22 includes in series the grid leak and condenser 23 and coil 24 completing the circuit to the cathode 2. The coils 2| and 24 in the grid circuits-are inductively coupled to the induction coils 3 and I in the plate circuits so as to provide a feed-back to th grid from the plate circuit as is common in oscillator circuits of the reverse feed-back type.

Each of the oscillator circuits has an additional load due to auxiliary circuit branches containing conductivity cells, one such circuit branch being connected in parallel to each of the plate circuits described above. Thus a circuit branch made up of a condenser 23 and in series therewith a test conductivity cell 4| is connected in parallel with the inductance 9 and condenser l3 to form the complete plate circuit based on plate 1. Similarly, the condenser 21 and in series therewith the standard conductivity cell 43 constitute a circuit branch connected in parallel with the inductance II and condenser II to form the complete plate circuit based on plate 3. As shown in Figure 1, in each case connection oi'the conductivity cell circuit branch to the cathode of the vacuum tube is completed through ground.

The condensers 23 and 21 are placed in the circuit to guard against the possibility of an accidental ground on windings 3 and i which are connected to the transformer windings l3 and I2, the voltage of which will usually be of the order oi 250 volts.

The physical form of the conductivity cell 4| containing the elements 33, 3| and 32 is the same as that described in Patent 2296367, issued on my copending case Serial No. 289,924. The construction of this cell can be better understood by reference to Figures 5 and 8 in which 33 indi cates an outer glass tube closed at the top, and at the bottom sealed to an inner glass tube 34 so as to form a closed annular space 33 between the tubes 33 and 34 while leaving the inner tube open at its lower end 33. At its upper end 31, the inner tube bends and passes out through the side wall or the tube 33. The end 31 of tube 34 is also open. The condenser plate 33 is in the sealed annular space 33 surrounding a midportion of the outer wall of the inner tube 34. A wire 33 is connected to the plate 33 and passes out of the tube 33 through the sealed nipple 33. A condenser in the conductivity cell circuit is thus formed by the plate 33, electrostatically coupled to the midpoint of the column 01' acid in the tube 34, through the glass wall of the tube 34. The resistances 3| and 32 shown in Figure 1 are the portions of the acid column (in tube 34) labelled 3| and 32 in Figure 5. Each of these portions of acid column joins the main body of This main The construction of the standard conductivity cell is also the same as that shown in Patent 2,296,867, issued on my copending case Serial No. 289,924. The construction of this cell may be better understood by reference to Figures 4, 6, and 7 m which 41 represents the outer glass tube of the conductivity cell 43. This tube is closed at bothends. Within tube 41 is an inner glass tube 43 extending only part of the length of tube 41, being sealed to the inner wall of tube 41 at 43, so as to form a closed annular space between tubes 41 and 43 while leaving tube 43 open to the portion of tube 41 lying below 43. In this annular space is the metal plate 43 arranged to surround the midportion of the outer surface of the inner tube 43. Inside the glass tube 43 is a column of standard acid. At its lower end the tube 43 expands and merges, as above described, at 'the point 49 with the outer tube 41, and the column of standard acid accordingly is separated from the outside at this point only by the single glass wall of the outer tube 41. The column of standard acid 44 thus, at its midpoint, forms a condenser with the metal plate 43, and at its lower end 43 forms another condenser with the grounded body or acid 43 which surrounds the conductivity cell 43.

The cell 43 thus includes in series a condenser, one side of which is a metal plate 43 and the other side of which is a column of standard acid separated from the metal plate by a glasswall which forms the dielectric for the condenser. The column 44 of the standard acid extending away from the plate 43 constitutes a resistance in series with the condenser 43. This column of liquid 44 at a point 43 remote fromthe condenser plate 43 is separated by a glass wall from the grounded body of process acid 43 in which cell 43 is immersed. The acid bodies. and 43 thus constitute the two sides of a condenser in which the glass wall is the dielectric. Since the body of acid 43 is grounded, the circuit is complete to the cathode of the vacuum tube, which is grounded. The wire 32 connects the condenser plate 43 to the condenser 21 01' Figure l.

The conductivity cell 43 contains additional electrodes whose purpose is to compensate for stray capacity eflects that might be introduced by variations in the upper level 01' the liquid column 44 due to thermal expansion, by formation oi a conducting liquid film along the glass surface above the level of the liquid, and possibly by the accumulation of condensed moisture or acidin the upper air spaces within the tube. These compensating means are'fully described in Patent 2,296,867, issued on my copending case Serial No. 289,924. Electrode 33, grounded by means of wire 33, is a cylinder of wire mesh or metal plate and is positioned on the inner surface of the outer glass tube 41 at its upper end. Another metal electrode 3| is arranged to surround the outer surface of the inner glass tube 43 at its upper end. The level of the standard acid is arranged to remain within the area surrounded by the electrode 3|.

The conductivity cells 43 and 4| are positioned side by side in an enlarged section of the pipe through which the process acid flows. This is shown in Figure 9 where the process. acid, the concentration oi which it is desired to control. enters the enlarged section where the conductivity cells are placed at 33 and leaves at 34. The process acid thus flows through the inner tube 34 of the test cell 4|, entering this tube at 33 and leaving at 31, to form the resistances 3| and 32,

and to form the condenser with plate 30: the process acid also surrounds the lower portion of the standard conductivity cell 40 to form one side of one of the condensers in the standard cell circuit. This body of process acid 46 surrounding the standard cell and flowing through the test cell is' grounded through contact with the grounded pipe through which it flows. The wire 52 from the standard cell 40 and the wire 38 from the measuring cell II are connected to the bridge network, as shown in the wiring diagram of Figures 1 and 3, and as indicated in Figure 9.

The transformer secondary l thus excites the oscillator circuit with which the test conductivity cell is associated while the transformer secondary l2 excites the oscillator circuit with which the standard conductivity cell is associated. As above pointed out, these circuits are alternately excited. As shown in Figure 1, a .direct current milliammeter 63 with middle zero scale is shunted between the ends of coils l0 and I2 and a potentiometer or variable resistance having the two variable arms 60 and SI -in series is also connected between these ends of coils ill and I2. The resistance armsGil and ii are varied by 'a movable contact 62 which is grounded.

Coils l0 and I2 are generally of identical design. Therefore, if both oscillator circuits take the same current, the milliammeter may be brought to a zero reading by connecting the center of the variable risistance to ground. When the currents in the two oscillator circuits are not the same, the milliammeter may be brought to a zero reading by proper adjustment of the movable contact 62 in the variable resistance. The bridge system of Figure 1 may thus be initially adjusted to the null reading of the indicator 63 while sulfuric acid of the desired concentration is in the test cell ll. Thereafter, as the process acid departs from this desired concentration, the resistances 3| and 32 will change and the current taken by the oscillator circuit with which they are associated will change accordingly. This change in current in the oscillator circuit will cause deflection of the needle in indicator 63.

The indicator shown in Figures 2 and 3 of the drawings is of the type described in Patent 2,296,867, issued on my copending application Serial No. 289,924. It is an induction type of instrument containing a disc, preferably of aluminum, which has its periphery slightly eccentric to the shaft and is positioned in the fields of the two coils l3 and It in such a way that these coils, when current flows through the oscillating circuits containing them, produce torques tending to turn the disc in opposite directions. A pointer and scale may be employed, as shown in the drawings. to indicate the differential torque on the disc, which differential also indicates the difference in load on the two oscillator circuits.

the oscillator circuit based on plate I is connected in series with the coil l3 of the indicator, while the secondary coil l2 which excites the oscillator circuit based on plate 8 is connected in series with the coil H of the indicator. The

other ends of coils l2 and I4 have a common tivity cells 40 and ll.

connection 84 which is a movable contact point in a potentiometer 85. This movable contact point is generally at approximately the point of zero potential in the potentiometer. The potentiometer 65 is energized by an additional secondary coil 88 coupled to the transformer primary 5. The center point of coil 66 is grounded.

Thus, if the oscillators energized by coils II and I2, respectively, are taking the same current..

the indicator 15 may bebrought to a zero reading by placing the movable connection 64 in the potentiometer at the point of zero potential which would be the center of the potentiometer, if of symmetrical design.

In the operation of the bridge system of Figure 2, the desired standard acid and the desired quality of process acid are placed in the two conduc- The movable connection 64 in the potentiometer is then adjusted-to obtain a zero reading on the indicator I5. Thereafter, as the concentration of process acid departs from the desired concentration, this departure will vary the current taken by the oscillator circuit based on coil l0 which in turn will vary the torque by coil l3 and thus cause a deflection in the indicator I5.

In the design shown in Figure3 of the draw-.

ings, the conductivity cells are not directly connected as circuitbranches in parallel with the plate circuits of the oscillators. Instead, each conductivity cell is contained in an auxiliary circoupled to the inductance ll of the oscillator circult on that side, the circuit between coil 42 and cell 40 being completed through ground.

Thus, part of the impedance of each oscillator circuit is due to "the auxiliary circuit coupled thereto and the amount of impedance reflected into the oscillator circuit by its associated auxiliary circuit depends upon the impedances in the auxiliary circuit. Thus the impedance in the oscillator circuit based on plate I (and accordingly the current taken by this circuit) depends in part upon the resistances 8| and 32 of the test conductivity cell contained in the auxiliary circuit inductively coupled to the oscillator circuit of plate I and as the resistances 3| and 82 change with variation in concentration of the sulfuric acid, this change will be reflected as a change in impedance in the associated oscillator circuit and the current taken by this oscillator circuit will vary in turn. As the current in the oscillator circuit varies, the torque exerted by coil l3 will vary and the needle in indicator i I! will be deflected. j

In the design shown in Figure 3, the indicator is brought to a zero reading when sulfuric acid of the desired concentration is in the conductivity cell ll by adjustment of variable capacities in the high-frequency portion of the oscillator circuits. Thus, in the design shown in Figure 3 of the drawings, plate I is connected to the cathode by a second plate circuit containing the fixed condenser 25 and the variable condenser 28 in series, and plate 8 is connected to the oathode through an additional plate circuit containing the fixed condenser 21 andthe variable connetwork formed. by the two oscillator circuits may be increased.

As indicated above, electric contacts may also be provided, positioned to be contacted by a member attached to the indicator, whereby, process equipment, e. g., a motorattached to a valve controlling theproportion of ingredients that make up the process material, as shown in Figure 9 of the drawings, may be actuated in accordance with deflections of the needle in the indicator. In the construction shown in Figure 9, concentrated acid enters at 56 and water for diluting this acid to a desired concentration enters at; 51. These ingredients are thoroughly ,mixed in mixing chamber 59. 58 indicates a valve connected to a motor armature for controlling the proportion of water entering the sulfuric acid. This motor may be actuated by a valve actuating circuit which is responsive-to the bridge network indicator,- and as above stated the valve actuating circuit may include means .providing a time lag after each successive actuation of the valve to permit time for the conductivity cells to reflect accurately the altered concentration of the aciddue to the opening or closing of the yalve. The time lag obviates "hunting by the control system.

' of automatic regulation, the valve actuating circuit referred to above and indicated in Figure 9 .of the drawings may, if desired, be omitted and the water valve controlled'manually in accordance with the readings either visually noted or recorded on the indicator of the bridge network.

It should be evident my device andmethod are not limited to the control 01' sulfuric acid or other liquid concentration in accordance with conductivity measurements. My device and method may be applied to controlling the quality of any desired material which is in the course of preparation or processing, in accordance with some electrical characteristic of the material. As above stated, in controlling the quality. of the material in accordance with its electrical properties, it is most advantageous to use an electrical bridge'network where the material in process may be compared with a standard material, and a differential indication of variations of the process material from the standard material may be obtained. As further stated above, it has been found advantageous in determining anelectrical characteristic of a corrosive chemical .product, or a material subject to electrolytic decomposition, to employ high frequencyalternating currents, both for minimizing electrolytic decomposition of the material and in order to make it possible to maintain. metallic circuit elements such as electrodes out of contact with the material. My device and method therefore are of great advantage in controlling the quality of such materials, where the use of two high frequency alternating current circuits in a bridgenetwork is desired.

Since certain changes in the constructions and process set forth above, which embody the invention, may be made without departing from its scope, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim: 1. A device for determining the quality of a material in process relative to a standard material by comparing electrical characteristics of V the process material and the standard material,

said device comprising two radio frequency alternating current circuits, means for energizing each of the two circuits alternately so as to maintain a radio frequency current in only one circuit at a time, a circuit element including a sample of the process material associated with one of the radio frequency alternating current circuits so as to consume a part of the energy in that circuit which is dependent upon the said electrical characteristic of the process material, a circuit element including a sample of the standard material associated with the second of the radio frequency alternating current circuits so as to con-- sume a part of the energy in the said second circuit which is dependent upon the saidelectrical characteristic of the standard material, an electrical instrument associatedwith the two radio frequency alternating current circuits for alternate energization of the instrument byv each circuit ,in proportion to the above described consumption of energy by the said circuit element in that circuit, and an element in said electrical instrument tending to respond oppositely to said energization of the instrument by each of the two radio frequency circuits, said element having a time lag in said response sufllciently greater than the period of alternate energization by the two radio frequency circuits that the resultant response of said element indicates an average difdevice comprising two radio frequency alternating current circuits, means for energizing each of the two circuits alternately so as to maintain a radio frequency current in only one circuit at a time, said means alternating the operation of the two circuits at a frequency of at least about 25 cycles per second, a circuit element including a sample of the process material associated with one of the radio frequency alternating current circuits so as to consume a part of the energy in that circuit which, is dependent upon the said electrical characteristic of the process material, a circuit element including a sample of the standard material associated with the second of the radio frequency alternating current circuitsso as to consume a part of the energy in the said second circuit which is dependent upon the said electrical characteristic of the standard material,

an electrical instrument associated with the tworadio frequency alternating current circuits for alternate energization of the instrument by each circuit in proportion to the above described consumption of energy by the said circuit element in that circuit, and In element in said-electrical instrument tending to respond oppositely to said energization of the instrument by each of the two radio frequency circuits, said element having a time lag in said response sufllciently greater than one twenty-fifth of a second that the resultant response of said element indicates an average,

difierence between the energization of the electrical instrument by the two radio frequency circuits. r

3 A device for determining thequality, of a material in process relativelto a standard material by comparing electrical characteristics of the process material and the standard material, said device comprising two radio frequency alternating current circuits, means for energizing each of the two circuits alternately so as to maintain a. radio frequency current in only one circuit, at a time said means alternating the,

operation of the two circuits at a frequency of at least about 25, cycles per second, a circuit element including a sample of the process mate, rial associated with one of the radio frequency alternating current circuits so as to consume a part of the energy in that circuit which is 1 dependent upon the said electrical characteristic of the process material, a circuit element including a sample of the standard material associated -with the second of the radio frequency alternating current circuits so as to consume. a

part of the energy in the said second circuit which is dependent upon the said electrical characteristic of the standard material, an

electrical instrument associated with the two than the period of alternate energization by the two radio frequency ircuits that the resultant response of said element indicates an average difference between the energization of the electrical instrument by the two radio frequency circuits, and means for adjusting the reading of the electrical instrument to a null point when the material in process is of a desired tion being in proportion to the above described consumption of energy by the material associated with said circuit, and said instrument tending to respond oppositely to the said energizations of the instrument by each of the two oscillator circuits and having a time lag in such response sufficiently greater than the period of alternate enerization of the two oscillator circuits that the resultant response of said instrument indicates an average difference between the energization of the instrument by each of the oscillator circults.

5. A bridge system for determining the quality of a liquid in process relative to a standard liquid by comparing the electricalconductivity of the liquid in process with the conductivity of the standard liquid, comprising two oscillator circuits the electrodes of which are contained in a twin thermionic tubecomprising a cathode common to both circuits, means for energizing each of the two oscillator circuits alternately so as to maintain a radio frequency current'in only one circuit at a time, an auxiliary circuit comprising a. column of the process liquid as a resistance element in the circuit, this auxiliary circuit being inductively coupled to one of the oscillator circuits to furnish an impedance and to consume a part of the energy of said oscillator circuit which is dependent upon the conductivity of the columnof process liquid, a secondauxiliary circuit "comprising a column of standard liquid as a resistance element in the circuit, this second auxiliary circuit being inductively coupled to the second of the oscillator circuits to furnish an impedance and to consume a part of the energy in said oscillator circuit'which is dependent upon the conductivity of the column of standard liquid, and anelectrical instrument associated with the two oscillator circuits for alternate energization by each of the two oscillator circuits during the time the radio frequency current is flowing in that circuit, said energization being in proportion to the above described consumption of energy in said circuit, said instrument tending to respond oppositely to said ,energizations by each of the two oscillator circuits and having a time lag in said response sufllciently greater than the r period of alternate energization of the two oscilquality, without affecting the frequency of either a of said radio frequency circuits.

4. A bridge system for determining the quality of a material'in processrelative to a standard material by comparing electrical characteristics of the proces material and the standard material, comprising two oscillator circuits, with one of which a sample of the process material is associated to furnish an impedance and to consume a part of the energy of said oscillator alternate energization of the instrument by each circuit during the time the radio frequency current is flowing in said circuit, said energizalator circuits that the resultant response of said instrument indicates an average diiference between the energization of the instrument by each of the two oscillator circuits.

6. A bridge system for determining the quality of a material in process relative to a standard material by comparing electrical characteristics of the material in process and the standard material, comprising two oscillator circuits in one of which a sample of the process material furnishes impedance and consumes a part of the energy in said circuit which is dependent upon its said electrical characteristic and in the other of which a sample of the standard material furnishes impedance and consumes a part of the energy in said circuit which isdependent upon its said electrical characteristic, means for energizing each of the two oscillator circuits alternately so as to maintain a radio frequency current in only one circuit at a time, said means alternating the operation of the two circuits ate-frequency in the range of about 25, to 600cycles per second, an electrical instrument connected to the two oscillator circuits for alternate energization by each of the two oscillator circuits during the time the radio frequency current is flowing in that circult, said energization being in proportion to the above described consumption of energy in said circuit, and an element in said instrument tending to respond oppositely to said energizations by each of the two oscillator circuits and having a time lag in said response sufliciently greater than one twenty-fifths! a second that the resultant response of said element indicates an average difference between the reaction of the element to each or said energizations of the instrument.

7. In a bridge network for determining the quality of a liquid in process relative to a standard liquid by comparing the electrical conductivity of the liquid in process withthe conductivity of the standard liquid, two reverse feedback oscillator circuits, the electrodes of which are contained in a twin tri'ode comprising a heated cathodejcommon to both circuits and separate grids and plates'for each circuit, the two oscillator circuits having approximately the same frequency of oscillation, means for energizing each of the two circuits alternately, said means including a low frequency alternating current, a standard conductivity cell connected to furnish impedance and consume a part of the energy in a branch of the plate circuit of oneoi the oscillator circuits, said cell containing a column of standard liquid electrostatically coupled to a conductivity elec-r trode positioned adjacent to the liquid column but separated therefrom by anon-conducting wall, a a

test conductivity cell connected to fumish 1m pedance and consume apart or theenersy in a branch of the platecirouit of the second of the oscillator circuits, this test conductivity cell containing a stream of liquid in process electrostatif cally coupled to a conductivity electrode positioned adjacent to the stream of process liquid but separated therefrom by anon-conducting wall, an

electrical instrument associated with thetwo oscillator circuits for alternate energization by each of the two oscillator circuits during the time a radio frequency current is flowing in that circult, said energization of the instrument by each oi said oscillator circuits being in proportion to the above described consumption of energy in that circuit, an'element in said instrument tending to respond oppositely to said'energization by each or the two oscillator circuits; said element having a time lag in said response suiliciently greater than one twenty-fifth of a second that the result-j ant response or said element indicates an average diflerence between the reaction oi. the element to each or the energizations oi theelectrical instrument, and a potentiometer associated with 

